1. Technical Field
The present invention relates to an ultrasound diagnosis apparatus, and in particular to an ultrasound diagnosis apparatus used for diagnosis of a hard tissue such as a bone.
2. Related Art
Simple quantitative measurement of mechanical characteristics such as bone strength is desired for diagnosing bone metabolic diseases such as osteoporosis, for judging fracture risk, and for quantitatively diagnosing bone union after treatment of bone fracture.
The evaluation of bone formation and bone union depends largely on X-ray photography, but quantitatively diagnosing bone strength by means of X-ray photography is very difficult. As a method of measuring bone strength in the related art, there is known a strength test of a sample bone of a measurement target. However, this method requires an extraction operation for obtaining a sample bone, and the method is thus invasive. A method of measuring an amount of bone and a bone density has employed devices such as general-purpose X-ray CT and DXA (dual-energy X-ray absorptiometry). However, these devices are merely means for measuring the amount of bone and cannot provide an evaluation of bone strength. Moreover, in light of the fact that tissue is irradiated with X-rays in these methods, these methods cannot be considered non-invasive.
Other attempts to quantitatively evaluate bone strength include a strain gauge method in which a strain gauge is mounted on an external fixator and the strain of the external fixator is measured; a vibration wave method in which a vibration is applied to a bone from the outside and a characteristic frequency is evaluated; and an acoustic emission method in which acoustic waves generated by a bone which has reached yield stress are detected. These methods, however, suffer from various problems in that a limitation is imposed on the treatment to which these methods can be applied, that the bone is subjected to invasion, and that evaluation precision is insufficient.
In view of the above circumstances, the inventors of the present invention have proposed an ultrasound diagnosis apparatus for noninvasively and quantitatively evaluating the mechanical characteristics of bone (refer to, for example, Japanese Patent Publication JP 2004-298205 A).
The ultrasound diagnosis apparatus described in JP 2004-298205 A forms a plurality of ultrasonic beams on a bone, obtains a plurality of echo signals corresponding to the individual ultrasonic beams to specify a surface point corresponding to the bone surface for each echo signal, and generates shape data of the bone surface on the basis of the plurality of surface points obtained from the plurality of echo signals. Then, a mechanical characteristic of the bone is evaluated on the basis of a change in the shape data when an external load is applied to the bone. Thus, the apparatus is an epoch-making technology capable of noninvasively and quantitatively evaluating the mechanical characteristics of a bone in a living organism from the shape data of the bone surface on the basis of the echo signals.
The inventors of the present invention have further improved the epoch-making technology described in JP 2004-298205 A and have studied methods of more precisely evaluating the mechanical characteristic of a hard tissue such as the bone. In particular, the present inventors have studied evaluation of the bone as a viscoelastic structure in consideration that the bone which is a representative example of a hard tissue is a viscoelastic structure simultaneously having an elastic characteristic and a viscous characteristic.